The present invention relates in general to nonlinear optical components and devices, and to lasers and more particularly to laser equipment in which the fundamental wavelength of an input laser energy of a solid state or gas or vapor laser is converted to different output wavelengths using nonlinear optical crystal components.
Laser systems are widely used in applications that include materials processing, tissue treatment and surgery, spectroscopy and defense applications. Laser systems operating at various fundamental wavelengths are advantageous for different types of operations in the following fields of use and others: materials processing, medical treatment and surgery, spectroscopy, defense, and scientific applications.
Different radiation wavelengths are desired for different applications. The radiation spectrum of most solid state lasers is relatively narrow with radiation output peaks occurring at fairly defined wavelengths. Output at the fundamental wavelength of a solid state laser oscillator is restricted by the availability of crystal and glass lasing media that are doped with available dopant ions.
Methods currently exist for generating additional wavelengths by converting the wavelength of a fundamental laser output to different wavelengths.
One technique for generating an output radiation beam having a different wavelength than that generated by the lasing medium is by the use of nonlinear frequency conversion crystals. Specialized nonlinear optical (NLO) crystals have been developed for use with currently available lasing media to provide an output wavelength different from the characteristic wavelength generated by the lasing medium itself. For example, U.S. Pat. Nos. 3,949,323 and 4,826,283, which are hereby incorporated by reference, disclose techniques for fabricating a harmonic crystal for use with lasing media where the crystal is responsive to an input fundamental wavelength to produce an output harmonic wavelength. Crystals useful for frequency conversion include the following types: Potassium titanyl phosphate (KTP or KTiOPO4), Lithium triborate (LBO or LiB3O5), Beta-barium borate (BBO), Potassium titanyl arsenate (KTA) and similar derivatives of KTP, lithium niobate (LiNbO3) and magnesium-doped LiNbO3 (MgO:LiNbO3), Lithium iodate (LiIO3), KNbO3, Zinc germanium phosphide (ZGP, ZnGeP2), silver gallium selenide (AgGaSe2, AGSe) and others. A more complete discussion of nonlinear devices and crystals used in such devices can be found in W. Koechner, Solid-State Laser Engineering (2d ed. 1988) and R. L. Sutherland, Handbook of Nonlinear Optics) 1996.
In anisotropic crystal, the direction of wave vector for an extra-ordinary wave is not generally the same as the direction of the beam propagation (Poynting vector). The extraordinary beam is said to walk-off the axis of the wave vector direction. Therefore, the ordinary and extraordinary beams of finite size will not completely overlap over the full length of a non-linear optical (NLO) crystal. The angle ρ is called the walk-off angle and can be of the order of a few degrees. The efficiency of critically phase-matched frequency conversion is strongly dependent on walk-off because beams that do not physically overlap cannot interact. Large walk-off angle can also affect the output beam quality.
When lights propagate along one of the principle axes of a NLO crystal, walk-off is vanished. Phase-matching along principle axis is possible for NLO frequency conversion at a certain laser wavelength, this is called 90°-phase matched or non-critically phase matched (NCPM) frequency conversion. 90°-phase matched (or NCPM) frequency conversions have the advantages of (i) Zero spatial walk-off between the ordinary and extraordinary polarized laser beams, therefore, long crystal might be used in the frequency conversion process for high conversion efficiency; (ii) Large angular acceptance, therefore, insensitive to angle misalignment or tight focused laser beam might be used.
Due to the above advantages, non-critical phase matching is an ideal phase matching condition for NLO frequency conversion. However, limited by nature, most of the NLO frequency conversions need to be critical phase matched, only at particular wavelengths non-critical phase matching might happen in some of the NLO crystals.
Accordingly, it is desirable to provide optical components and devices that overcome the above and other problems.